The present invention relates generally to the field of Digital Subscriber Line (DSL) systems, and particularly to packetized voice access systems.
Conventionally, telephone service is provided by the Public Switched Telephone Network (PSTN), which consists of a multiplicity of telephone switches interconnected by various transport media and formats (the PSTN transport network), and connected to telephone terminals at subscriber's premises by twisted pair loop wires.
FIG. 1 illustrates a conventional circuit switched telephone network, represented generally by the numeral 10. In this example, the PSTN transport network 12 interconnects telephone switches 14. The switches, in turn, interface by means of POTS line cards 16 to individual twisted pair loops 18 and telephone terminals 20. In the case of typical residential telephone service, the POTS line card 16 provides a number of functions required to operate a telephone terminal 20 connected at the subscriber's residence. These functions are often collectively referred to as BORSCHT and usually include, but are not limited to:                Battery—supplying power;        Overvoltage—protecting the line card against environmentally caused overvoltages;        Ringing—supplying a signal which energizes the telephone terminal receiver;        Supervision—detecting whether a subscriber telephone receiver is on hook or off hook;        Coding—converting the analog loop signal into digital representation;        Hybrid—separating the received from the transmitted signals; and        Termination—terminating the loop with a required standard electrical impedance at the line card.        
Although implementation details differ, the B, O, R, S, and T functions typically reside in the Analog Front End 22 and the C, H, and T functions in the Voice Engine 24. The line card 16 supplies power to the telephone terminal 20 from an uninterruptible power supply 26. The power supply 26 is physically situated in the telephone central office and is referred to as the Central Office (CO) Battery. The CO Battery plant is engineered and sized to supply power to conventional telephone terminals 20 even in the event of a general power outage. Therefore, telephone service is characterized by a high degree of availability, a grade of service referred to as “lifeline” POTS. Typically, a line card 16 is capable of supplying up to 30 mA of current at 48 V to a twisted pair loop 18 and telephone 20. However, the exact value of current depends on the combined electrical resistance of the twisted pair loop 18 and the telephone terminal 20. The exact current limit value is implementation dependent and may range from 20 to 35 mA.
With the explosive growth in data communications, a Multi Service Data Network (MSDN) has evolved which now has more aggregate bandwidth and is growing more rapidly than the PSTN transport network. Data communications networks are developing that more easily support protocols and network elements which enable the routing and switching of self-contained collections of bits known as packets or cells. Essentially, each packet/cell can be independently routed or switched by devices known as routers to different destinations based on a destination address contained within each packet or cell. This is known as packet switched networking. If a packet stream contains digital representations of a voice conversation, routing of these packets represents a function analogous to conventional circuit switched telephony switching. It should be noted that the term voice is used generically and its definition may be extended to include other analog transmissions such as fax, conventional modems, and the like. In the following descriptions, packets and cells are understood to be analogous entities and, unless specifically indicated, may be used interchangeably and individually without restricting the generality of the description. The terms VoIP (Voice over IP) and VoATM (Voice over ATM) are to be treated in a similar fashion.
FIG. 2 represents a network configuration for interconnection of a MSDN to a conventional telephone by means of voice packetization technology, namely, a Packet Switched Voice Access Network 30. The Multi Service Data Network 32 interconnects and routes packets/cells among various Digital Subscriber Loop Access Multiplexers (DSLAMs) 34, which multiplex data streams from multiple DSL Line Cards 36. Each DSL line card 36 interfaces to and communicates over a twisted pair loop 18 to DSL Customer Premises Equipment (CPE). The DSL CPE may be self contained and connected to a source of packet/cell data such as a computer (not shown), may be a DSL Network Interface Card (DSL NIC) (not shown) internal to a computer, or may be a CPE herein called a Voice Packetizing CPE 38. The DSL line card 36 may communicate using any of a number of methods that impress a signal representing a digital bit stream onto twisted pair loops 18 more conventionally used to carry POTS signals. These methods include, but are not limited to, the methods or services generally known as ADSL (Asymmetric DSL), SDSL (Symmetric DSL), HDSL (High-rate DSL), VDSL (Very high-rate DSL), ISDN (Integrated Services Digital Network), MVL (Multiple Virtual Lines), or CAP (Carrierless Amplitude/Phase).
The Voice Packetizing CPE 38 transforms the analog signals from a conventional telephone terminal 20 into voice packets or cells suitable to be carried in a DSL signal on the twisted pair loop 18 from the subscriber's premises. In order to operate a conventional telephone terminal 20, the Voice Packetizing CPE 38 includes most of the functions associated with a conventional telephone switch POTS line card 16 in its Voice Engine 42 and Analog Front End 44. Some specific differences from the corresponding POTS line card blocks might include coding to a 16 bit linear digital representation rather than Claw or A-law, the provision of 24 V rather than 48 V powering of the telephone terminal, reduced robustness to line overvoltages, and the like. The differences do not impact on the nature or significance of functions described below. A Voice over Internet Protocol (VoIP) or Voice over Asynchronous Transfer Mode (VoATM) Engine 46 may include circuits and algorithms for reducing the bit rate of the digitized voice stream received from the Voice Engine 42. The VoIP/VoATM Engine 46 also processes the reduced bit rate stream into packets or cells and may also include an echo cancellation capability for reducing the subjective audible effects of the delay introduced by the coding and packetizing operations. These functions typically implement a set of protocols which may include an embodiment of ITU Recommendation H.323, G.168 or similar standards and algorithms. The packet/cell stream generated by the VoIP/VoATM Engine 46 is transmitted to and received from the DSLAM 34 by means of the DSL modem 40. The DSL modem 40 may implement any of a variety of digital subscriber loop standards, such as ITU Recommendation G.992.1 (“Full rate ADSL”) or G.992.2 (“ADSL lite”).
Because the circuits contained within the Voice Packetizing CPE 38 require more power than can typically be provided by a Line Card 36, they are typically powered by means of a power supply 48 which derives its power from the local AC mains. If a local power outage disrupts AC mains power, communication using the Voice Packetizing CPE is not possible until AC mains power is restored. This is problematic for subscribers accustomed to telephone service being available despite power outages, and especially in case of a life or security threatening emergency situation. Uninterruptible AC power supplies (UPS) are commercially available which will provide AC power for a short time in the event of a local AC mains power outage. However, the disadvantages of local UPSs are the purchase cost, the limited time these devices can supply power, and the regular maintenance that is required to ensure they are operational in the event of a power outage.
Further, in conventional packet based networks, provision of lifeline packet voice service requires up to 3 separate pieces of equipment: a DSLAM, POTS Access Equipment such as a Digital Loop Concentrator, and a Gateway. The Gateway is an item of network equipment that provides a translation and mediation of voice and signaling signals and protocols between the PSTN and the MSDN. The need for three separate pieces of equipment increases overall network and equipment complexity and complicates service provisioning and maintenance.
What is needed is a mechanism to obviate or mitigate at least some of the above disadvantages.